The Philippines has accused China of aggressively colonizing the South China Sea, as fears of a major conflict between the two countries grow. 

It comes as a U.S. carrier strike group, led by the USS Theodore Roosevelt, returned to the area for the second time in less than two months.

Over the last couple of weeks, around 220 Chinese paramilitary ships, manned by maritime militias, have "swarmed" around a disputed reef in the South China Sea, with the Philippines warning that the "incursions" could "trigger unwanted hostilities" -- the toughest remarks yet from Manila.

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China maintains that the vessels are simply fishing boats, sheltering in the area due to poor sea conditions -- but they have done no fishing and the weather has been good. They also switch on powerful lights at night. The Philippines' government says the vessels are part of China's maritime militia and manned by reservists operating under the orders of the Coast Guard and People's Liberation Army (PLA).

The boats are moored at the Whitsun Reef within Manila’s 200-mile exclusive economic zone, and there are fears the Chinese ships may try to claim the reefs. The Philippine defense minister has warned that Beijing is also planning to occupy and assert its control over more disputed territory.

Philippine President Rodrigo Duterte expressed concern to China’s ambassador about Chinese vessels massing in the South China Sea, and Vietnam also urged Beijing to respect its maritime sovereignty. On Tuesday, Japanese Foreign Minister Toshimitsu Motegi expressed strong concerns to his Chinese counterpart about the incursions.

The reef is part of the Spratly Islands, one of the main disputed archipelagos in the South China Sea, some 200 miles west of the Philippine province of Palawan. The Philippines says the offshore region is part of its territory, but it is claimed entirely or in part by China, Brunei, Malaysia, Taiwan and Vietnam.

An international tribunal invalidated China’s claim to 90% of the South China Sea in 2016, but Beijing does not recognize the ruling. China has built islands in the disputed waters in recent years, putting airstrips on some of them. Taiwan, Malaysia, Vietnam, the Philippines and Brunei all claim parts of the sea.

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Meanwhile, the USS Theodore Roosevelt has also sailed into the South China Sea to conduct "routine operations," marking the second time it has shown up in the disputed waters in less than two months. The carrier group entered the South China Sea on April 4. The unit will conduct various exercises while in the area, ranging from anti-submarine drills to "coordinated tactical training."

At the same time, the first-ever Chinese aircraft carrier group has started exercises near Taiwan and has said such drills will become regular. China's Navy said the carrier group, lead by the Liaoning -- the country's first aircraft carrier put into active service -- was carrying out "routine" drills in the waters near Taiwan.

The aim is to "enhance its capability to safeguard national sovereignty, safety and development interests," it said. "Similar exercises will be conducted on a regular basis in the future," the Chinese Navy added, without elaborating.

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China's statement follows Taiwan's Defense Ministry reporting of a new incursion by China's Air Force into the island's air defense identification zone on Monday. It said it had a "full grasp" of the situation in the air and at sea surrounding Taiwan and that it was "appropriately handling" the matter. 

Taiwan President Tsai Ing-wen, meanwhile, is overseeing a revamp of the island’s military, rolling out new offensive equipment such as "carrier killer" stealth corvettes in an attempt to deter any Chinese aggression.